Gas turbine airfoill

ABSTRACT

An airfoil ( 1 ) comprises internal cooling air passages ( 15   a,b,c ) arranged in a serpentine manner with one or more radially outward and radially inward extending passages ( 15   a,b,c ). The passages ( 15,   b,c ) are in fluid connection by turns ( 16, 20 ) of approximately 180°. According to the invention, the turns near the platform of the airfoil connecting a radially inward extending passage with a radially outward extending passage is realized by a root turn ( 20 ) defined by the passage sidewalls ( 15   d,e ), which extend radially inward to the radially inner end of the root section ( 2 ) of the airfoil, and by an end plate ( 21 ) attached to the radially inner ends of the walls ( 15   d,e ).

FIELD OF INVENTION

[0001] This invention relates to a gas turbine airfoil with internalserpentine passages for cooling purposes.

BACKGROUND ART

[0002] Turbine airfoils are subjected to the very high temperatures ofthe hot gas driving the turbine. In order to prevent damage to theairfoils due to the high temperatures and assure a reasonable lifetimethe airfoils are cooled externally and internally by a cooling medium,typically cooling air bled from the compressor of the gas turbine.Internal cooling of the airfoil is realized by several passages withinthe airfoil between the pressure sidewall and the suction sidewall ofthe airfoil. The passages typically extend spanwise from the root of theairfoil to its tip. Some of the passages consist of a single passagewith an exit port near the tip of the airfoil and/or several filmcooling holes on the edge or on the side wall of the airfoil. Otherpassages follow a serpentine path allowing the cooling air to flow forexample from the root to the tip and around a 180° turn. From the tip itextends towards the root and around a further 180° turn that directs itagain toward the tip where it finally exits through exit ports or filmcooling holes. Serpentine cooling passages of this type are disclosedfor example in U.S. Pat. No. 5,403,159. They allow for a high internalheat transfer with a minimum amount of cooling air.

[0003]FIG. 1 shows a radial cross-section of a typical airfoil I of thestate of the art with several internal passages extending radiallyinward and outward between a root section 2 and a tip 3. A firstinternal passage 4 extends from an entry opening 5 in the root section 2radially outward to the tip 3 of the airfoil. Cooling air can flow fromthe root section 2 through the passage and exit via several coolingslots 6 along the trailing edge 7 as well as through a tip hole 8. Asecond internal passage 10 extends from an entry opening 11 radiallyoutward along the leading edge 12 of the airfoil. Cooling air flowsthrough this passage 10 and exits via a tip hole 13 and through severalrows of film cooling holes 31 drilled through the leading edge 12 of theairfoil. A serpentine passage comprises an entry opening 14 at theradially inner end of the root section, a first passage 15a extendingradially outward with a tip hole 17. At the tip a 180° turn 16 leads toa passage 15 b extending radially inward. At the radially inner end ofthe passage a second 180° turn 18 leads to a third passage 15 cextending radially outward to a tip hole 19. Cooling air flowing throughthe straight and serpentine passages cool the airfoil from within byimpingement cooling and exits through the film cooling holes on theedges of the airfoil 1 and/or through the tip holes. Other typicalairfoils have several serpentine cooling passages or serpentine passagescomprising five passages with four turns.

[0004] Airfoils with internal serpentine geometry for the coolingpassages are typically manufactured by an investment casting process,which utilizes a ceramic core to define the individual internalpassages. Following the casting the ceramic core is removed from theairfoil by a leaching process. The film cooling holes on the edges andsidewalls of the airfoil are then realized by a laser drilling process.This process involves, previous to the actual drilling, the insertion ofa backing or blocking material which limits the laser radiation to thedesired locations of the film cooling holes and prevents damage to thepassage walls and other inner surfaces of the airfoil. Such a method isdisclosed for example in U.S. Pat. No. 5,773,790. It uses a wax materialas a blocking material

[0005] During the process of casting the internal passages it is oftendifficult to maintain the separation of the passages in the cores due tothermal strains caused by differential heating and cooling rates of thecore and surrounding metal.

[0006] A current practice to maintain the separation of the serpentinepassages 15 a,b,c and to support the core during the casting processutilizes conically shaped features in the core. These conical featuresare formed as part of the core and extend from the root section throughan opening in the wall of the 180° turn 18 and into the passages 15 band 15 c. After the part is cast and the core is leached out the conicalfeature is closed off with a spherically shaped plug 30 that is brazedinto place.

[0007] The conical feature maintains a near constant cross-sectionalarea and outer radius of the outer wall through the 180° turn in orderto minimize pressure loss. Typical measured pressure losses through theturns are usually >1.5 times the dynamic pressure of the cooling airstream entering the turn.

[0008] However, the conical feature presents a weak spot in the corewhere it can break resulting in movement of the passages 15 b and 15 c,the turn 18, and the root section of the passage 15 a.

[0009] Following the casting process and the leaching out of the corematerial a backing material must be inserted into the cooling passagesfor the laser drilling of film cooling holes. As the passages 15 b and15 c following the 180° turns are not easily accessible from either end,it is difficult to fill these passages with backing material. In currentpractice this problem is circumvented with the use of a liquid wax,which is typically hot injected into the opening 14 until wax is seenexiting from the tip hole 19. After the completion of the laser drillingthe waxen backing material is removed from the airfoil by heating theairfoil and burning the wax. This practice has shown however, that theuse of wax as a backing material does not sufficiently absorb the laserenergy and therefore provides only limited protection from so-calledback wall strike.

SUMMARY OF THE INVENTION

[0010] It is an object of this invention to provide an airfoil and amethod to manufacture this same airfoil that comprises internal coolingfluid passages arranged in a serpentine path having one or more radiallyoutward extending and one or more radially inward extending passages,which are connected by turns of approximately 180°. In particular, it isthe object of the invention to provide an airfoil and a method tomanufacture such an airfoil that enables improved maintenance of thepassage separation during casting and the use of a blocking material fora laser drilling process that provides greater shielding compared to theblocking material used in current manufacturing methods.

[0011] An airfoil comprises internal cooling air passages arranged in aserpentine manner having one or more radially outward extending passagesand one or more radially inward extending passages and turns ofapproximately 180° providing fluid connection between a radially inwardand a radially outward extending passage. A radially inward extendingpassage in a serpentine passage is defined by the inner surfaces of thepressure and suction sidewall of the airfoil, a first wall and a secondwall separating it from neighboring passages. A radially outwardextending passage in a serpentine passage following the radially inwardextending passage in the fluid flow direction is defined by the innersurfaces of the pressure and suction sidewall of the airfoil, theaforementioned second wall, and a third wall. This third wall can be aseparating wall to a further cooling passage or the leading or trailingedge wall of the airfoil.

[0012] According to the invention each radially inward extending passageis in fluid connection with the next radially outward extending passagein the direction of the cooling fluid flow by means of a root turn. Thisroot turn is defined by the first wall of the radially inward extendingpassage and the third wall of the radially outward extending passage,which both extend to the radially inner end of the root section of theairfoil. The root turn is further defined by a member that closes offthe root turn at the radial inner end of the root section of the airfoiland is attached to the radially inner ends of the first and third wallsof the passages.

[0013] The radially inward extending passage and the radially outwardextending passage are thus combined below the airfoil platform. There istherefore no need for a curved outer wall for a turn at the level of theairfoil platform. This geometry for an airfoil avoids a 180° turn with acurved side wall and avoids all together the necessity of a hole forinsertion of the conical features and the subsequent closing with a ballbraze.

[0014] In a method for manufacturing the airfoil according to theinvention a ceramic core is used for the casting of the internalpassages between the pressure and suction sidewall of the airfoil. Bymeans of the ceramic core the root turn at the radially inner end of theroot section of the airfoil is printed out in the casting process.Following the casting process, the ceramic material is leached out ofthe cast airfoil. Thereafter, strips of PTFE or Teflon are inserted intothe passages and film cooling holes are drilled. During the drillingprocess the Teflon strips protect the surrounding cast material from thelaser radiation. After the drilling process the Teflon strips areremoved and a member is placed at the end of the root turn in order toclose it off.

[0015] During the casting process the root turn is open ended allowinggood access for the application of additional core supports in theregion of the turn. This provides improved control over the wallseparation and the location of the turn. Following the casting processand removal of the core the open turn enables excellent access for theinsertion of a backing material necessary to perform the laser drillingof film cooling holes.

[0016] This backing material is not necessarily fluid such as a waxenmaterial but instead can be of a stiffer material such as strips ofTeflon (PTFE). Teflon provides an improved shielding compared to thewaxen material used in current practices. Following the laser drillingprocess the Teflon strips are again easily removed upon which each rootturn is closed off With a member such as an end plate welded or brazedto the radially inner ends of the sidewalls of the serpentine passagescombined by the root turn.

[0017] Furthermore, the open root turns enable an improved leaching outof the core material after the casting process.

BRIEF DESCRIPTION OF THE FIGURES

[0018]FIG. 1 shows a spanwise cross-section of an airfoil of the stateof the art and manufactured by current practice,

[0019]FIG. 2a shows a spanwise cross-section of an airfoil according tothe invention with several root turns connecting radially inwardextending cooling passages with radially outward extending passages,

[0020]FIG. 2b shows a spanwise cross-section of an airfoil according tothe invention with a greater number of internal passages and root turns.

DETAILED DESCRIPTION OF THE INVENTION

[0021]FIG. 1 is described as part of the state of the art above.

[0022]FIG. 2a shows an airfoil 1 of similar construction as in FIG. 1.It extends from the root section 2 to the tip 3 and comprises severalinternal passages. A single pass passage 4 extends from the opening 5for cooling air at the root 2 radially outward to the tip 3. Cooling aircan exit through cooling slots 6 along the trailing edge 7 and a tiphole 8. A second single pass passage 10 extends along the leading edge12 from the entry opening 11 to the tip where it comprises a tip hole 13for the cooling air. A three pass serpentine passage consists in thisexample of the invention of an entry opening 14 for the cooling air atthe root of the airfoil, passages 15 a and b, which are in fluidconnection by a first 180° turn 16 and passage 15 c, which is in fluidconnection with passage 15 b by the second 180° root turn 20. Theradially inward extending passage 15 b is formed by the inner surfacesof the pressure and suction sidewalls of the airfoil and a first wall 15d which separates it from the upstream and radially outward extendingpassage 15 a, and finally by a second wall 15 f which separates it fromthe downstream and radially outward extending passage 15 c. According tothe invention, the 180° root turn 20 extends to the radially inner endof the root section 2 as far as the level of the entry openings 5, 11,and 14. It is formed by the inner surfaces of the suction and pressuresidewalls and the radially inward extensions of the walls 15 d and 15 e.The radially outward extending passage 15 c of the serpentine passage isdefined on one side by the sidewall 15 e, which separates it from thecooling passage 10.

[0023] The ceramic core used for the casting of the airfoil as shownallows for improved precision maintenance of the wall separation duringthe casting. Furthermore, the open turn and therefore lack of blindpassages allows the use of and easy access for the insertion ofadditional core supports. The pressure loss around the root turn 20 forthis airfoil is approximately the same as that for the airfoil of thestate of art shown in FIG. 1.

[0024] After the casting process and the removal of the ceramic core byleaching, firm sheets of Teflon or PTFE are inserted as backing materialfor the laser drilling. The firm Teflon sheets provide a greaterprotection of the airfoil material surrounding the film cooling holes tobe drilled compared to the wax material used in current practices.

[0025] The root turn 20 is finally closed off by means of an end plate21, which is welded or brazed to the radially inner end of the passagewalls 15 d and 15 e of the passages 15 b and 15 c.

[0026]FIG. 2b shows a similar airfoil as the one in FIG. 2a. Thenumerals in this figure refer to the same components as those in FIG.2a. The airfoil comprises an additional three pass serpentine passage 26a,b,c with walls 26 d,e,f a root turn 25. The first root turn 20combines the radially inward extending passage 15 b and the radiallyoutward extending passage 15 c. The second root turn 25 combines thepassage 26 b and 26 c. The root turn 25 is completed by an end plate 22attached to the radially inner ends of the walls 26 d and 26 e of thepassages 26 b and c.

[0027] The root turns according to the invention are applicable to notonly airfoils with one or more three-pass passages as in FIGS. 2a and 2b but also to airfoils with five and multipass passages. Terms used inthe Figures 1 airfoil 2 root section 3 tip 4 first internal passage 5entry opening 6 cooling slots 7 leading edge 8 tip hole 9 10 secondinternal passage 11 entry opening 12 trailing edge 13 tip hole 14opening for cooling air to enter 15a first radially outward passage ofserpentine 15b radially inward passage of serpentine 15c second radiallyoutward passage of serpentine 15d wall of serpentine passage 15e wall ofserpentine passage 15f wall of serpentine passage 16 turn 17 tip hole 18turn 18′ outer wall of turn 19 tip hole 20 root turn 21 end plate 25root turn 26a first radially outward passage of serpentine 26b radiallyinward passage of serpentine 26c second radially outward passage ofserpentine 26d wall of serpentine passage 26e wall of serpentine passage26f wall of serpentine passage 29 opening for cooling air to enter 30spherically shaped plug 31 leading edge film cooling holes

1. An airfoil (1) comprises internal cooling air passages (15 a,b,c, 26a,b,c) arranged in a serpentine manner with one or more radially outwardextending passages (15 a,c, 26 a,c), one or more radially inwardextending passages (15 b, 26 b) and turns (16 18, 20, 25) ofapproximately 180°, by which the radially inward extending passages (15b, 26 b) are in fluid connection with the radially outward extendingpassages (15 a,c, 26 a,c), whereas in each serpentine passage (15 a-c,26 a-c) a radially inward extending passage (15 b, 6 b) is defined bythe inner surfaces of the pressure and suction sidewall of the airfoil,a first wall (15 d, 26 d) and a second wall (15 e, 26 e), whichseparates the radially inward extending passage (15 b, 26 b) from aradially outward extending passage (15 a,c, 26 a,c) that follows theradially inward extending passage (15 b, 26 b) in the direction of thecooling air flow, and a radially outward extending passage (15 c, 26 c)is defined by the inner surfaces of the pressure and suction sidewall ofthe airfoil, the second wall (15 e, 26 e) of the radially inwardextending passage (15 b, 26 b), and a third wall (15 f, 26 f)characterized in that the turn (20, 25) providing fluid connectionbetween a radially inward extending passage (15 b, 26 b) and the nextradially outward extending passage (15 c, 26 c) in the direction of thecooling fluid flow is defined by the first wall (15 d, 26 d) and thethird wall (15 e, 26 e), where the first and third walls (15 d,e, 26d,e) extend radially inward to the radially inner end of the rootsection (2) of the airfoil (1), and in that the root turn (20, 25) isclosed off at the radially inner end of the root section (2) by means ofa member (21, 22) connecting the ends of the first and third walls (15d,e, 26 d,e).
 2. Airfoil (1) according to claim 1 characterized in thatthe root turn (20, 25) is closed off by an end plate (21, 22) that isattached to the radially inner ends of the first wall (15 d, 15 e) andof the third wall (15 e, 26 e) of the serpentine passage (15 a-c, 26a-c).
 3. Method of manufacturing an airfoil (1) according to claim 1wherein a ceramic core is used for the casting of the internal passagesof the airfoil, by which means the root turn (20, 25) extending to theradially inner end of the root section of the airfoil is printed out inthe casting process, the ceramic material is leached out of the castairfoil, and strips of Teflon are inserted into the passages through theopen ended root turn (20, 25), and film cooling holes are formed in alaser drilling process, where the strips of Teflon provide protectionfrom the laser radiation, and the Teflon strips thereafter removed fromthe internal passages, and an end plate (21, 22) is attached to theradially inner ends of the first and third walls (15 d,e, 26 d,e) of theinternal passages in order to close off the root turn (20, 25).